Commitments and Contingencies - Leases (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2021		Sep. 30, 2020		Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 28	[1]	$ 29	[1]	$ 86	[2]	$ 86	[2]
Finance Lease, Right-of-Use Asset, Amortization	2		2		6		5
Finance Lease, Interest Expense	4		4		12		13
Finance Lease, Cost	6		6		18		18
Lessee, Operating Lease, Liability, Payments, Due	529				529
Total minimum obligation	466				466
Interest component of obligation	(68)				(68)
Finance Lease, Liability, Undiscounted Excess Amount	(340)				(340)
Operating Lease, Liability	461				461
Finance Lease, Liability	126				126
Less current portion	(95)				(95)				$ (97)
Less current portion	(4)				(4)
Operating lease liabilities	366				366				$ 432
Noncurrent operating and finance lease liabilities	122				122
Short-term Lease, Cost					1		1
PPAs
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	26		26		77		75
Lessee, Operating Lease, Liability, Payments, Due	468				468
Interest component of obligation	(61)				(61)
Operating Lease, Liability	407				407
Other
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	2	[3]	$ 3	[3]	9	[4]	$ 11	[4]
Lessee, Operating Lease, Liability, Payments, Due	61				61
Interest component of obligation	(7)				(7)
Operating Lease, Liability	$ 54				$ 54

[1] PPA capacity payments are included in electric fuel and purchased power on the consolidated statements of income. Expense for other operating leases is included in O&M expense and electric fuel and purchased power.

[2] PPA capacity payments are included in electric fuel and purchased power on the consolidated statements of income. Expense for other operating leases is included in O&M expense and electric fuel and purchased power.

[3] Includes immaterial short-term lease expense for 2021 and 2020.

[4] Includes short-term lease expense of $1 million for 2021 and 2020, respectively.